1. Field of the Invention
The invention pertains to an ultrasonic detector and more specifically to a light weight, battery-operated, precision instrument for detecting ultrasonic sound or vibrations, which is useful in detecting leaks and malfunctions.
2. Prior Art
It is well known that ultrasonic generators and detectors can be used to locate leaks or defects, e.g. in pipes. Such a system is shown in U.S. Pat. No. 3,978,915 to Harris. In that arrangement, ultrasonic generators are positioned in a chamber through which the pipes pass. At the ends of these pipes, exterior to the chamber, ultrasonic detectors are located. At the point where a leak occurs in the pipe or the pipe wall is thin, the ultrasonic energy will enter the pipe from the chamber and travel to the end of the pipe where the detector is located. The detector will receive an ultrasonic signal at the end of the pipe indicating the existence of the leak or weak spot in the pipe.
Ultrasonic sensors have also been used to detect ultrasonic energy generated by friction within mechanical devices as disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al. The greater the amount of friction, the greater the intensity of the generated ultrasonic energy. Applying a lubricant to the device reduces friction and consequently the intensity of the generated ultrasound drops. Measuring ultrasonic energy thus provides a way to determine when lubrication has reached the friction generating surfaces. Additionally, faulty devices such as bearings generate a higher level of ultrasonic energy than do good bearings, and thus this condition can also be detected. However, conventional means require two people to perform this procedurexe2x80x94one person to apply the lubricant to the device, and one person to operate the ultrasonic detector.
Since ultrasonic energy used for these purposes is generally in the range of 40 kHz, it is too high in frequency to be heard by a human being. Thus, means are typically provided for heterodyning or frequency shifting the detected signal into the audio range, and various schemes are available for doing this.
By locating an ultrasonic generator in a closed chamber, a standing wave pattern with peaks and nodes is established. If a node occurs at the position of a leak or weak spot, no ultrasonic energy will escape and the defect will not be detected. One method of addressing this problem is disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al. Goodman teaches varying the frequency of the applied ultrasonic energy so that the position of the nodes will shift over time so that a leak at a null or node will be detected. However, resort to this method adds complexity and expense to the testing hardware.
Ultrasonic transducers generally produce a low voltage output in response to received ultrasonic energy. Thus, it is necessary to amplify the detected signal using a high-gain preamplifier before it can be accurately processed. However, if low cost heterodyning and display circuitry are to be used, means must be made available to attenuate the amplified signal to prevent saturating these circuits when high input signals are present. This attenuation also adjusts the sensitivity of the device. For a hand-held unit, the degree of attenuation should be selectable by the user.
For example, U.S. Pat. No. 4,785,695 to Rose et al. discloses an ultrasonic leak detector with a variable resistor attenuator used to adjust the output level of an LED bar graph display. However, this attenuation method does not provide a way to establish fixed reference points to allow for repeatable measurements.
U.S. Pat. No. 5,089,997 to Pecukonis discloses an ultrasonic energy detector with an attenuation network positioned after an initial preamplifier and before the signal processing circuitry, which creates an audible output and an LED bar graph display. The resistors in the Pecukonis attenuation network are designed to provide an exponential relationship between the different levels of attenuation. However, Pecukonis does not heterodyne the detected signals to produce an audible output but rather teaches the benefits of a more complex set of circuits which compress a broad range of ultrasonic frequencies into a narrower audible range. For many applications, the cost and complexity of this type of circuitry is not necessary.
In addition to detecting ultrasonic sound escaping from a leak or defect, a detector using an acoustic transducer must be able to accurately locate the source that sound. To this end, conical sound collectors are used in conjunction with the transducer to increase its directionality as illustrated in U.S. Pat. No. 4,287,581 to Neale, Sr. However, conventional detectors do not utilize collection cones which are also specifically designed to provide additional input signal gain of an amount consistent with the units of measure provided by the detector.
When using ultrasonic energy to detect leaks, it is useful to have a portable ultrasonic sensor which indicates the presence and intensity of ultrasonic energy both visually and audibly. Goodman discloses an ultrasonic sensor which displays intensity of the detected signal on an output meter operable in either linear or logarithmic mode and also provides for audio output through headphones. U.S. Pat. No. 4,987,769 to Peacock et al. discloses an ultrasonic detector which displays the amplitude of the detected ultrasonic signal on a ten-stage logarithmic LED display. However, the detector disclosed in Peacock does not process the detected signal to produce an audible response, nor does it provide for signal attenuation after the initial pre-amplification stage.
The present invention is directed to providing a versatile, light weight, battery-operated, precision instrument for detecting ultrasound or vibration. The invention provides a wide dynamic range as well as fixed reference points through the entire range to provide for repeatable measurements. In one embodiment, the detector circuitry can be housed in a handheld unit used to detect leaks. In another embodiment, the detector can be used with a grease gun to detect whether a sealed mechanical device such as a bearing, gear box, or transmission has been properly lubricated and if the device is faulty.
In an illustrative embodiment of the invention, the ultrasonic detector has an acoustic or contact ultrasonic transducer for detecting ultrasonic sound and converting it into an electric signal whose amplitude and frequency reflect that of the detected sound. A removable focusing probe funnels ultrasound into the transducer so as to collect the ultrasonic energy and focus it on a single transducer crystal. Prior art devices may use a plurality of transducers placed on a focusing surface. However, there are nulls in the reception with such an arrangement. Thus, the probe eliminates the possibility of missing a leak caused by receiving and/or positioning nulls that occur with multi-transducer receiving transducer modules. This probe also provides an additional 10 dB of signal gain.
The output signal is passed through a high-gain preamp which has significant headroom so as to avoid saturation when large ultrasonic energy fields are detected. Further, the preamplifier is arranged as a charge amplifier so it will be relatively insensitive to changes in transducer output impedance or capacitance. The preamplifier""s output is capacitively coupled to a logarithmic attenuator network in order to adjust for baseline or ambient noise levels and to prevent saturation of the output audio amplifier and display circuitry. The network provides fixed signal attenuation levels of 0 dB through xe2x88x9270 dB in xe2x88x9210 dB steps . A supplemental variable resistor is also provided to allow for additional attenuation when extremely high ultrasound intensity levels are present.
The attenuated signal is then fed into a heterodyning circuit where the ultrasonic frequency is shifted into the audible range, filtered and amplified. Heterodyning is achieved by commutating or multiplexing the inputs of a tuned audio frequency filter with a high frequency signal, then differentially filtering and amplifying the heterodyned audio output, which then drives the speaker output and bar graph display. This technique takes advantage of the differential inputs of the LM386 audio amplifier to provide additional carrier rejection as well as a doubled voltage swing.
The audio amplifier of the present invention provides a low impedance output so the heterodyned and amplified signal can be used to drive 8 or 16 ohm speakers or headphones at a fixed output level. This output may also be used as an input for signal processors or analyzers.
The audio amplifier also drives a logarithmic LED bar graph display. This display has ten bar segments calibrated at 3 dB per bar in order to provide an output display covering a 30 dB window. The use of the logarithmic attenuator in combination with the log output display converts the input signal to a decibel format. This combination also gives the detector a wide dynamic range of approximately 100 dB in 33⅓ fixed discrete steps. These steps provide a set of fixed, repeatable reference points anywhere within the display range of the instrument. An additional 10 dB of dynamic range can be obtained by removing the focusing probe if necessary.
In a second embodiment, the detector is attached to a lubrication tool such as a grease gun for use in detecting when a mechanical device has been properly lubricated. When dealing with sealed bearings, for example, there is normally no way to know when the proper amount of lubricant has reached the friction areas, such as the raceway and the ball bearings. Too much grease can build up the internal pressure in the bearing and cause damage or can xe2x80x9cblowxe2x80x9d the seal, allowing contaminants to get into the bearing. Too little grease can cause the bearings to overheat and seize due to excess friction. The same is true for other sealed mechanical devices such as gear boxes or transmissions. Ultrasonic energy can be detected directly via the grease gun coupling. Alternatively, the ultrasonic transducer can be attached to the grease gun coupling but acoustically isolated from it and provided with a separate acoustic waveguide for transmitting ultrasonic energy from the sealed device directly to the transducer assembly.
Devices such as sealed bearings, gear boxes or transmissions generate ultrasonic energy when in use as a result of internal friction. As lubrication is applied, the internal friction is reduced. Consequently, the intensity of the generated ultrasound is lower. Measuring the level of ultrasound generated by the device as it is lubricated thus provides a way to determine when enough lubricant has been applied.
Additionally, devices such as sealed bearings generate higher than normal levels of ultrasound when they begin to fail. Because of the detector""s fixed, repeatable reference points and wide dynamic range, the amount of ultrasound a specific lubricated bearing generates can be precisely measured and compared to a base line level. The degree to which the measured level exceeds normal indicates how badly the bearing has degraded.
Attaching the detector to a lubrication tool such as a grease gun has the further advantage of allowing the lubrication and ultrasound measurement procedures to be performed by one person, instead of the two required when conventional means are used.